How is back-monitor doing? Is it stable enough in OpenLDAP 2.1 to use
in a critically important LDAP server?

All known issues have been fixed. I usually enable it at compile time
and check its contents every now and then, but I'm not currenlty using
it in any production environment. However, there's been postings
about its use which seen to give quite positive a feedback.

Stanford has used the monitor backend in production since April 2003. We
currently use it to collect statistics for our 10 production systems, and
it works quite well. We have used back-monitor in both 2.1 and 2.2 series.